PROJECT SUMMARY/ABSTRACT With a new generation of tobacco products either on the market or being developed, surveillance is critical to describe the changing patterns and correlates of tobacco use in the U.S. population. The Population Assessment of Tobacco and Health (PATH) Study is the first large research effort undertaken by the National Institutes of Health (NIH) and the Food and Drug Administration (FDA). PATH is a national longitudinal study of tobacco use and health that is following approximately 46,000 U.S. household residents ages 12 and older. Westat is the research company leading the development, data collection, and analyses of PATH. Building on systems that are already in place at Westat, this proposal seeks to estimate the test-retest reliability and validity of the PATH questionnaires. A separate reliability and validity study of the PATH adult and youth round four instruments is proposed in which a national sample of 875 respondents will be interviewed in a subsample of the PATH sample areas. Respondents will be interviewed twice, with the re-interview completed within 10 to 17 days after the initial interview. For a subset of the items on which discrepancies are found, the questionnaire will include probes designed to shed light on the nature of the discrepancy. To assess the validity of answers to key items on tobacco use saliva specimens will be analyzed for biomarkers and, at the end of the reinterview, the interviewer will photograph the products used by the participant and they will be compared to the brands self-reported in the questionnaire. Additional methodological enhancements and analyses to broaden the scientific value of the study will be conducted to identify person-level variables associated with differing levels of reliability, identify characteristics of the questions that affect the reliability of the answers, and examine the convergence of different methods for estimating reliabilities. PATH will serve researchers, agencies, and policy makers with the most comprehensive and up-to-date data on the changing tobacco and substance use landscape of any survey to date. It is, thus, crucial to establish the reliability of the PATH data. The proposed team includes experienced survey methodology experts and tobacco control specialists. The project will advance knowledge of person-level predictors of reliable tobacco survey responses as well as characteristics of the survey items themselves that may produce more reliable and valid reporting; these findings will be incorporated in new tools to predict the reliability of survey questions, advancing measurement practices in the study of substance use behaviors.